1. Field of the Invention
This invention relates to a screwdriver having a provision for holding a screw in place on its tip before the screw is installed or after it is removed, more particularly, to such a screwdriver having a hexagonally-shaped tip for engaging a screw having a screw having a hexagonally-shaped socket in its head, and to such a screwdriver configured for installing and/or removing screws installed in bone during surgical procedures.
2. Description of the Related Art
In many surgical procedures, such as the repair of fractured bones, screws of various types are driven into bone. While these screws are often left in place, sometimes they must be removed. In the process of installing or removing such screws, it is of particular importance that the screw not be dropped from the screwdriver, as it is often difficult to find a screw lost within the surgical site. In many such applications, the screw threads cut mating threaded surfaces in bone during the installation of a screw. Thus, it is also important to maintain a tight relationship between the screwdriver and the screw itself during the process of installing the screw, so that such threaded surfaces can be accurately cut, without unnecessarily widening the threaded surfaces being cut or otherwise weakening the threaded bone through wobbling movements occurring between the screwdriver and the screw. Furthermore, the screwdriver should be easily and completely releasable from the screw under control of the user, so that it can be removed from an installed screw without weakening the attachment of the screw within fragile bone.
The patent literature includes a number of examples of screwdrivers operating with socket-headed screws having a capability to hold and release the screws while providing the required driving torque. For example, U.S. Pat. No. 5,025,688 describes a fastener drive tool for applying a rotational torque to a threaded fastener for driving the fastener into or out of a workpiece. The drive tool has an elongated shaft portion with a free end which is selectively controllably engageable with a recess formed in the fastener. The free end is formed with a fastener engaging expansion portion. The expansion portion is operated by a draw shaft being selectively lockable in a bit retaining position. The expansion portion of the bit portion creates substantial fastener retaining forces on the opposing internal surfaces of the fastener recess in which it is inserted.
In a first embodiment of this prior-art fastener drive tool, the expansion portion is an elastomeric disk having a central hole through which the draw shaft extends. When the draw shaft is pulled upward, an enlarged head portion of the draw shaft axially compresses the elastomeric disk, causing it to expand radially into engagement with a socket within the screw head. The engagement torque and holding forces between the screwdriver and the screw are thus limited by the radial forces which can be generated within the elastomeric disk in this way, and by a reduction of the overlapping engagement distance between the socket driving surfaces extending above the elastomer and the socket. This distance is reduced by the presence of both the enlarged head portion of the draw shaft and of the elastomeric disk within the screw head socket.
In a second embodiment of this prior-art fastener drive tool, the expansion portion comprises the tip, which includes a vertical slit and conical bore having its enlarged end at the end of the tool and its narrow end beginning at a cylindrical hole provided for the draw shaft. The draw shaft includes an enlarged conical end, which is pulled upward into the conical bore, causing the tip to expand at the vertical slit. Thus, the engagement torque which can be established between the screwdriver and the screw is limited by the inclusion of both the slot and the relatively large conical bore as spaces within the tip.
U.S. Pat. No. 4,779,494 describes a screw-gripping screwdriver including a handle, a shaft composed of two blade members, a sleeve which is disposed coaxially about the blade members, and a tip. The blade members have inclined surfaces which cooperate with the sleeve so that, during movement of the sleeve along the blade members, a camming action occurs so that a relatively large movement of the sleeve causes a relatively small movement of the blade tips. In one embodiment, the blade tip members have portions which are generally trapezoidal in shape suitable for use with a hexagonal opening in a hex head screw. However, the degree of precision with which the screw can be handled is limited by the flexibility of the blade members, which are flexed by the torque used to turn the screw. Furthermore, the screw is released from the screwdriver by pulling the sleeve so that the camming action is reversed, causing a movement of the blade tip members that may tend to loosen a screw fastened into fragile bone tissue.
U.S. Pat. No. 4,581,962 describes an invention comprising basically a combination of a barrel and collar, preferably made of stiff but flexible plastic, which fit over and enclose a tool for threaded fasteners, either with a fixed bit or one which can accept insertable bits. The barrel provides at its working or tip end a set of flexible elements which act to grip screw heads and hold them registered with the tool. The barrel can be fixed in place on the tool shaft by a partial turn of either form of collar. Collar works with a spring to move the barrel into gripping position. A variation for jewelers' screwdrivers uses a miniature barrel without a collar but with a spring. This method is limited by a requirement that both the socket in the screwhead and the peripheral surface of the screwhead must be controlled within tight limits to allow proper engagement of the tool.
U.S. Pat. No. 5,056,387 describes a screw-holding screwdriver having a sleeve threadedly attached to its shank. A chuck on the end of the sleeve has a slot in the side to receive a screw head and shank. By screwing the sleeve up on the shank, the screwdriver bit engages the screw head and clamps the screw head in the chuck for driving the screw. When partially inserted, the sleeve is rotated on the shank to release the screw to permit removal of the chuck from the screw. In this way, the screw is firmly retained while driving. However, the use of this screwdriver is limited by the fact that the distal portion of the chuck extends between the bottom of screwhead and the outer surface of the material into which the screw is being driven. Before the screwhead can be driven to the surface of the material, the screwdriver must be removed from the screwhead so that the drive bit can be extended through an opening in the lower portion of the chuck. In an operation requiring the driving of a screw into fragile bone, or the removal of a screw from such bone, it is desirable to be able to grip the head of the screw throughout the attachment or removal process.
Other examples of patent literature, such as U.S. Pat. No. 4,827,812, describe screwdrivers having interchangeable bits without mechanisms for releasably holding the screws to be driven or removed using the interchangeable bits. What is needed is a single mechanism for both holding an interchangeable bit in place on the end of a screwdriver and for releasably holding a screw in place on the interchangeable bit.
The present invention, which is described in detail below, overcomes the various disadvantages of the prior art, as described above.